Magnetic fields are useful in modern technology. For example, Magnetic Resonance Imaging (MRI) systems produce medical diagnostic images, and nuclear magnetic resonance (NMR) systems are increasingly useful for chemical analysis. Such systems employ large high-field magnets.
Many large magnets generate external fields which can interfere with the operation of instruments in the vicinity, as well as produce personnel hazards. For example, cathode ray tube displays commonly used with computers are distorted by fields as low as 0.6 Gauss. People wearing cardiac pacemakers are at risk when exposed to magnetic fields of intensities that could interfere with a cardiac device. Also, strong magnetic field gradients can attract and accelerate nearby ferromagnetic objects, creating personnel hazards.
Magnetic shields are used to contain the external fringe fields of large magnets. For example, magnetic shields contain the external field of MRI magnets when computer displays are used in the vicinity. Magnetic shields are also used to produce an internal region of low magnetic field. For example, electron microscopes and photomultiplier tubes generally use magnetic shields for protection from ambient magnetic fields.
The most popular form of magnetic shield is a shell, or enclosure, made of steel or other material with high magnetic permeability. MRI magnets, for example, are often installed within room-sized steel enclosures to contain the far field. Another version of MRI magnet has a steel flux-return cage built directly onto the magnet case. Magnetic shields of this type are extremely heavy and costly. The cost of such a shield may be comparable to the cost of the MRI magnet. The cost of any reinforcement needed in the building in which it is to be installed can also be high. A magnetic shield providing equal field attenuation with less material and lower construction costs would thus be desirable.
Present magnetic shields, such as those built around MRI magnets, generally reduce the fringing field to about 1 to 2 Gauss near the shield. This field is too high for most color displays to be used without severe color distortion. The effectiveness of the shields is therefore limited. It is not feasible to reduce the fringing field to the desired level of 0.5 Gauss by making the shield thicker because this would require many more tons of steel. In addition, most or all of the openings that are normally built into a shield would have to be closed with more ferromagnetic material in order to achieve 0.5 Gauss. Many shield applications would not be feasible without access to the interior volume through these openings. A new type of shield providing improved effectiveness while retaining the openings in the shield is therefore needed.
An additional problem of conventional shields has to do with the Earth's magnetic field. Any large volume of ferromagnetic material, such as a room-sized MRI shield, collects and focuses the ambient field of the Earth, producing local flux concentrations at the edges of the shield. Such flux concentrations may be large enough to distort computer displays. This effect is not eliminated by use of more ferromagnetic material in the shield, nor by use of material of higher magnetic permeability.
The weight, cost, and limited effectiveness of these systems have prompted other attempted solutions. MRI magnets have been built with two coaxial coils, the outer coil having opposite polarity from that of the inner coil and powered so as to cancel the far field. The nested coil approach succeeds in reducing the weight of the system relative to the weight of a steel-enclosed system, but at considerable extra expense due to the increased number of turns of conductor and the increased cooling requirements. Also, the central field of the two coils partially cancel, resulting in reduced performance of the MRI system.
The magnetic shielding problem has no generally acceptable solution, as shown by the design of mobile MRI systems, which are typically left unshielded. Consequently, the surrounding area is cordoned off, requiring large areas to be inaccessible for general use.
Common to all prior ferromagnetic shields is the leakage of flux into the shielded region. This penetration of magnetic field is due partly to the finite permeability of the material, and partly to the penetration of magnetic flux through various openings which are usually necessary in the shield.
A region of low magnetic field can be created by use of an array of electromagnets, oriented and powered so as to cancel the ambient field. Such an array may be practical for small scale application, but in dealing with high fields and large spatial volumes, sufficient coils would have to be so large and would consume so much electrical power that they would not be practical.
An array of permanent magnets may also be used to cancel the ambient magnetic field within a specified region. For large scale applications, this approach becomes impractical because of the cost of the permanent magnet material and because of the large magnetic forces between portions of the material.
Magnetic shields can be produced using superconducting foils. Certain superconductors, known as Type I superconductors, expel magnetic fields. Regions of low field can be created by cooling such foils below their superconducting transition temperature while in a folded configuration, and then opening the foils out to form a bag-like shape whose boundary, being the superconductor, prevents field penetration. Unfortunately, the shield must be maintained at a low temperature, of the order of a few degrees Kelvin, which is incompatible with most applications.
A new magnetic shield is needed, characterized by higher effectiveness, lower weight, and lower cost than conventional ferromagnetic shells, greater practicality than arrays of electromagnetic coils or permanent magnets, and requiring no cryogenic cooling. The shield should further avoid focusing the ambient magnetic field or other features incompatible with common applications.